I. Field of the Invention
This disclosure relates generally to titanium (Ti) alloys. In particular, alpha-beta Ti alloys having an improved combination of ballistic and mechanical properties achieved with a relatively low-cost composition are described as well as methods of manufacturing the Ti alloys.
II. Background of the Related Art
Ti alloys have found widespread use in applications requiring high strength-to-weight ratios, good corrosion resistance and retention of those properties at elevated temperatures. Despite these advantages, the higher raw material and processing costs of Ti alloys compared to steel and other alloys have severely limited their use to applications where the need for improved efficiency and performance outweigh their comparatively higher cost. Some typical applications which have benefited from the incorporation of Ti alloys in various capacities include, for example, aircraft components, medical devices, high-performance automobiles, premium sports equipment and military applications.
A conventional Ti-base alloy which has been successfully used in military systems is Ti-6Al-4V which is also known as Ti64. As the name suggests, these Ti alloys generally comprise 6 wt. % aluminum (Al) and 4 wt. % vanadium (V) with up to 0.30 wt. % iron (Fe) and up to 0.30 wt. % oxygen (O) typically included.
The development of Ti64 provided an alloy having an attractive combination of ballistic and mechanical properties for military ground vehicle systems. Military applications which implement a weldable wrought titanium alloy such as Ti64 as structural armor plate typically have strict compositional and performance requirements. For example, in a document entitled “Detail Specification: Armor Plate, Titanium Alloy, Weldable,” MIL-DTL-46077G, 2006 the U.S. Department of Defense identified provisions for four classes of Ti64 wrought titanium alloy armor defined by strict elemental composition ranges and density requirements, as well as minimum mechanical and ballistic properties. With regard to Ti alloy-based armor plate, the goal is therefore to provide Ti alloys which meet or exceed established standards while minimizing the associated raw material and processing costs.
A number of approaches have been followed in attempting to produce Ti alloys having the required combination of properties at reduced cost. For example, Ti alloys have been produced by electron-beam single-melting (EBSM). This approach has made the manufacture of Ti alloys more cost-effective and enabled their implementation in additional military systems. Another approach focused on the substitution of a quantity of iron (Fe) in place of vanadium (V) as a beta stabilizer in the Ti alloy to reduce raw material costs as disclosed, for example, by U.S. Pat. No. 6,786,985 to Kosaka, et al. (hereinafter “Kosaka”). However, the Ti alloy developed by Kosaka required the inclusion of molybdenum (Mo).
Yet another approach has involved developing Ti alloy compositions which permit processing from ingot to final mill product at temperatures entirely within the beta-phase region of the alloy as disclosed, for example, in U.S. Pat. No. 5,342,458 to Adams, et al. (“Adams”). Adams states that the higher ductility and lower flow stresses which exist at higher temperatures in the described alloys minimize surface and end cracking, therefore increasing yield. U.S. Pat. No. 5,980,655 to Yoji Kosaka and U.S. Pat. No. 5,332,545 to William W. Love disclose approaches wherein Ti64 alloys having improved mechanical and ballistic properties were formed by increasing the oxygen concentration beyond the ranges which were specified by standard military guidelines.
A number of Ti alloys having compositions analogous to Ti64, but with additional components included therein are also known in the art. These Ti alloys were developed to provide, among other things, low-cost high strength Ti alloys with acceptable levels of ductility. An example is provided by U.S. Pat. No. 7,008,489 to Paul J. Bania which, in one embodiment, discloses a Ti alloy having at least a 20% improvement in ductility at a given strength level. However, in addition to the base Ti—Al—V—Fe—O components present in Ti64, the disclosed alloy also includes concentrations of tin (Sn), zirconium (Zr), chromium (Cr), molybdenum (Mo), and silicon (Si). The large number of elements present in these alloys necessarily increases the raw material costs of the thus-formed Ti alloy.
Another example is provided by U.S. Patent Appl. Publ. No. 2006/0045789 to Nasserrafi, et al. (“Nasserrafi”) directed to Ti alloys that can be manufactured from recycled titanium. In one embodiment, Nasserrafi discloses a Ti alloy comprising Ti—Al—V; however, the alloy also includes one or more elements selected from the group consisting of Cr, Fe and manganese (Mn) in concentrations from 1.0 to 5.0 weight percent. The relatively high levels of Cr, Fe and Mn and low ductility limit the alloy's applicability to military systems. Each of the aforementioned patents and patent applications are incorporated by reference in their entirety as if fully set forth in this specification.
Despite the improvements from the standpoint of composition, properties and processing costs which have been attained to date, there is a continuing need to develop new and improved Ti alloys and associated manufacturing methods which achieve minimum mechanical and ballistic performance standards at continually lower cost.